1. Field of the Invention
The present invention relates to a soldering flux and a solder paste composition.
2. Description of the Related Art
In general, a solder paste composition used when electronic components are mounted on a substrate is produced by mixing a soldering flux with a solder alloy powder. When soldering is performed using the solder paste composition, a portion of the soldering flux tends to remain as flux residue near a solder joint on the substrate.
In general, the above soldering flux contains a base resin, an activator, and a solvent. As the base resin, rosin has been widely used.
Along with the recent improvement of the performance of electronic products and realization of high density of the electronic products, a substrate is often exposed under an environment where temperature difference is extreme. When flux residue formed from a soldering flux using rosin as a base resin is placed under such an environment, the flux residue becomes easily hard and brittle due to its nature. The flux residue is easily cracked with the elapse of time. Thus, since water easily permeates through a circuit portion through the crack, there occur such problems as short circuit of a circuit and corrosion of the metal of the circuit.
As a method of solving the above problem, there is a method of using an acrylic resin or the like as a base resin instead of a rosin-based resin or using the rosin-based resin and the acrylic resin together. For example, as disclosed in JP 2011-121059 A, there is a method of using as a base resin an acrylic resin, prepared by copolymerizing a monomer containing (meth)acrylic ester having a long-chain alkyl group, and rosins at a fixed compounding ratio.
Along with the recent miniaturization of component terminals, an opening of a metal mask used in a screen printing method, which is a method of supplying a common solder paste composition, tends to be reduced in diameter. In the screen printing method, the solder paste composition is filled in the opening of the metal mask by a squeegee. After the filling of the solder paste composition, the solder paste composition is transferred to the substrate side when the metal mask separates from the substrate. At this time, since the solder paste composition is adhered to the squeegee and a wall surface of the metal mask, there occurs such a phenomenon that the volume and shape of the solder paste composition transferred to the substrate side are not in accordance with the design of the metal mask.
In some cases, miniaturized components and large-sized components are mounted on a substrate while being mixed with each other. In this case, in order to maintain the amount of the solder paste composition to be supplied to electrodes of the large-sized components at a certain level or more, the thickness of the metal mask is often set to be not less than a constant thickness, and an aspect ratio of the opening of the metal mask corresponding to the miniaturized component terminals increases. Thus, under this state, abnormality in the transferred shape of the solder paste due to adhesion of the solder paste onto the wall surface of the metal mask tends to occur.
When the opening of the metal mask is reduced in diameter, a particle diameter of a solder alloy powder contained in the solder paste composition is required to be reduced. In this case, the surface area of particles of the solder alloy powder (hereinafter referred to as solder particles) increases with respect to the volume. Thus, active force of a flux enough to reduce an oxide film on a surface of the solder particles and melt the solder particles is required.